For example, there is a speaker device in which a yoke having an annular magnet and a center pole portion and a plate made of a magnetic material are included, and a voice coil wound around a coil bobbin is held by a magnetic gap formed between the center pole portion and the plate. In this type of speaker device, when the voice coil is energized, the coil bobbin changes (moves) in an axial direction of the center pole portion, and audio is output.
In addition, there is a speaker device, which is similar to the above-described speaker device, provided with an annular and elastic damper. Here, an inner circumferential portion of the damper is connected to an outer circumferential surface of a coil bobbin, and an outer circumferential portion of the damper is connected to a frame that functions as a casing. The damper has a function of holding a voice coil in a magnetic gap without touching a plate when the coil bobbin is changed.
Incidentally, the damper accounts for a certain weight ratio of the whole speaker device. Thus, the presence of the damper increases a weight of the speaker device and causes suppression of change of the coil bobbin and decrease in acoustic conversion efficiency. For example, the weight ratio of the damper to the whole speaker device is set to about 15% to 20%.
In this regard, there is a speaker device in which a predetermined portion is filled with a magnetic fluid instead of a damper, and a weight of the speaker device is reduced by omitting the damper, thereby improving acoustic conversion efficiency (for example, see Patent Document 1 and Patent Document 2).
A speaker device disclosed in Patent Document 1 has a configuration in which a magnetic gap at a position where a voice coil is present is filled with a magnetic fluid.
A speaker device disclosed in Patent Document 2 has a configuration in which a sub-magnetic circuit is included in addition to a main magnetic circuit, a sub-magnetic gap is formed in the sub-magnetic circuit, and the sub-magnetic gap is filled with a magnetic fluid to support a voice coil.